DESCRIPTION: Autopsies of Alzheimer's disease (AD) patients indicate the presence of organized deposits of B amyloid (AB) surrounding the microvasculature. These deposits are postulated to originate from amyloid precursor protein (APP) fragments released by activated platelets; the mechanism of this deposition found in patients with Alzheimer's disease but not in controls remains unclear. To date, it has not been shown whether APP fragments originating from platelets are capable of traversing the blood-brain barrier (BBB), and of then contributing to neural AB deposits. We have demonstrated the existence of an APP processing defect in platelets from patients with Alzheimer's disease leading to the retention of APP on the surface of activated platelets. The APP processing defect may be related to Aa plaque deposition in the cerebral vasculature. We have developed a model system, utilizing platelets and endothelial cells (EC), and have shown that human blood-brain barrier EC (BEC) express enzymatic activities capable of cleaving retained APP on activated platelets. Both control and AD-BEC express a and B-secretase-like proteases, with cells from Alzheimer's disease brains having an additional g-secretase-like activity. Additionally, we demonstrate the presence of platelet-derived Aa/APP fragments in the BEC layers of our model, supporting our hypothesis that platelets contribute to the cerebrovascular A\B and to the progression of Alzheimer's disease. We plan to pursue these findings by (1) studying the interactions between the endothelium and platelets by investigating the importance of our documented AD-specific APP processing defect in these interactions, utilizing a novel 4-layer model of the BBB, (2) investigating the passage of platelet-derived APP fragments and AB deposition in the underlying BEC and smooth muscle cell (SMC) layers of the cerebral vasculature and modulation of this transport by different isoforms of apo E, in particular apo E4 and (3) investigating the role of the serine and metallo-proteases present in BEC and SMC from autopsied brain of control and Alzheimer's disease patients, in facilitating the delivery of AB and APP fragments to the cerebrovasculature. Our goal is to determine the mechanism by which platelet-derived APP fragments or Aa are released, transferred through the sub endothelial layer of the cerebrovasculature and deposited between this layer and the SMC, in the hope of designing modalities to interfere with this process and slow the consequent progression of Alzheimer's disease.